Try this
% define an arbitray array a
a = [1, 2, 5, 6, 3, 2, 5, 8, 9, 1];
if length(a) <= 2
extreme = a;
else
extreme = zeros(1,length(a));
extreme(1) = a(1); % first element must be extreme value
count = 1; % count the number of extreme value
if a(2) > a(1)
increase = true; % use 'increase' to record the trend of increasing or decreasing
else
increase = false;
end
for n = 3:length(a)
if a(n) > a(n - 1)
if increase == false
count = count + 1;
extreme(count) = a(n - 1);
end
increase = true;
else
if increase == true
count = count + 1;
extreme(count) = a(n - 1);
end
increase = false;
end
end
extreme(count + 1) = a(end); % last elements must be extreme value
extreme = extreme(1:count + 1);
end
